The presence of large deposits of oil shale in the high plateau, semi-arid region of the western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil". A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents such as U.S. Pat. Nos. 3,661,423; 4,043,597; 4,043,598; and 4,192,554; and in U.S. Pat. application Ser. No. 070,319 filed Aug. 27, 1979, by Chang Yul Cha, entitled TWO-LEVEL, HORIZONTAL FREE FACE MINING SYSTEM FOR IN SITU OIL SHALE RETORTS. Each of these applications and patents is assigned to Occidental Oil Shale, Inc., assignee of this application, and each is incorporated herein by this reference.
These patents and applications describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort, or merely as a retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes igniting the upper level of the shale for establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone downwardly through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbons, and a residual carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off-gas is also withdrawn from the bottom of the retort. Such off-gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any portion of the gaseous retort inlet mixture that does not take part in the combustion process.
U.S. Pat. Nos. 4,043,597; 4,043,598; and 4,192,554 disclose methods for explosively expanding formation containing oil shale toward horizontal free faces to form a fragmented mass in an in situ oil shale retort. According to such a method, a plurality of vertically spaced apart voids of similar horizontal cross-section are initially excavated one above another within the retort site. A plurality of vertically spaced apart zones of unfragmented formation are temporarily left between the voids. A plurality of horizontally spaced apart vertical columnar explosive charges, i.e., an array of explosive charges, is placed in each of the unfragmented zones and detonated to explosively expand each unfragmented zone upwardly and/or downwardly toward the void or voids above and/or below it to form a fragmented mass having an average void volume about equal to the void volume of the initial voids. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
U.S. Patent application Ser. No. 070,319 discloses a method for explosively expanding formation containing oil shale toward a horizontal free face to form a fragmented mass in an in situ oil shale retort. According to such a method, a void having a horizontal cross-section similar to the horizontal cross-section of the retort being formed is initially excavated. A plurality of vertically spaced apart zones of unfragmented formation are left above the void. Explosive is placed in each of the unfragmented zones and detonated for explosively expanding such zones toward the void to form a fragmented mass in the retort having an average void volume about equal to the void volume of the initial void. The overlying zones can be expanded toward the void in a single round or a plurality of rounds. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
Past retorting techniques disclose that a combustion zone formed in a vertical in situ oil shale retort is advanced vertically downwardly through the fragmented mass. It has been disclosed that desirably such a combustion zone is flat, extends completely across the retort, and is advanced vertically downwardly in a planar wave.
Problems can be encountered, however, when using such techniques.
For example, it has been found that when formation is explosively expanded to form the fragmented permeable mass of formation particles in an in situ oil shale retort, it is sometimes difficult to completely fill the retort cavity with the fragmented mass. Thus, when explosively expanding formation toward a horizontal free face adjacent a "limited void", the available void volume may not be completely filled and a void space can remain over the fragmented mass in the retort. A "limited void" as used herein is defined as a void which has less available volume than would be required for "free expansion" of formation toward the void. When oil shale is explosively expanded toward an unlimited void, a certain maximum void fraction is present in the unfragmented mass, resulting from such "free expansion". When oil shale is expanded toward a "limited void", the void fraction can be no more than permitted by the available void space of the limited void and may be less due to interaction with unfragmented oil shale, for example. It is believed that with oil shale confined by surrounding walls and capable of expanding only to such a "limited void", gases from the detonation may not have full opportunity to act on the oil shale particles before such particles reach obstructions such as adjacent walls, a face opposite to the expanded formation, or oil shale expanding from the opposite sides of the void.
An in situ retort has, for example, been formed containing a fragmented mass of formation particles having a top surface mounded at about the center of the retort. This resulted in a relatively low void space between the fragmented mass and overlying formation at the center of the retort and a relatively high void space between the fragmented mass and overlying formation near the edges of the retort. This mounding effect is encountered when the blasting pattern comprises a square array of blastholes perpendicular to a horizontal free face toward which formation is expanded with detonation commencing near the center of the retort and progressing in bands generally radially outwardly from the center. Such a blasting pattern tends to cause formation to move toward the center of the retort, thereby resulting in the uppermost part of the mound near the center.
Since past techniques have disclosed that it is desirable that the combustion zone be flat and extend horizontally across the retort, preferably the entire top surface of the fragmented permeable mass is ignited for forming the flat combustion zone. Once the ignition process is complete, an oxygen-supplying gas is then introduced to advance the combustion zone through the retort.
Several problems are encountered with such a processing technique when the retort is not filled and the fragmented mass is mounded.
Firstly, having a void space above the fragmented mass in the region where the mass is ignited is a problem because heating of overlying unfragmented formation during the ignition process can cause thermal sloughing of formation into the void, thereby disrupting ignition. Secondly, the combustion zone formed is not flat, but tends to acquire the mounded or curved shape of the top surface of the fragmented mass. Time-consuming and expensive corrective measures can be required to flatten such a combustion zone.
In the past, in situ oil shale retorts have been formed with a drift communicating with the fragmented mass of formation particles through a side boundary near the bottom of the retort. This drift is used for withdrawal of the gaseous and liquid products of retorting, as well as for withdrawal of retort off-gas.
In such a retort, a combustion zone can be formed across the top of the fragmented mass and advanced vertically through the retort as described above. The retort inlet mixture, which is introduced into the retort to advance the combustion zone, is introduced through one or more inlets which extend vertically through unfragmented formation above the retort. When such an arrangement of retort inlets is used, i.e., when the retort inlet(s) extend vertically through unfragmented formation above the retort and the retort outlet extends through a side boundary at the bottom of the retort, an appreciable quantity of oil shale in the fragmented mass can remain unretorted at the end of the retorting operation. This problem can arise because when the gas outlet is in the form of a drift opening into the retort through its side boundary, gas flow tends to concentrate toward that side of the retort. This results in a combustion zone that does not remain flat and advance uniformly vertically through the retort, but one that advances more rapidly in the region of the retort adjacent the outlet. As a consequence of both the gas flow bypassing portions of the fragmented mass and the combustion zone advancing non-uniformly through the retort, an appreciable volume of oil shale adjacent the side boundary of the retort opposite the retort outlet can be bypassed. The bypassed oil shale can remain unretorted, thereby inhibiting the yield of gaseous and liquid products from the retort.
Additionally, such in situ retorts can be formed hundreds of feet below the ground surface. Therefore, in order to form the above described vertical inlets, a void is excavated above the retort and the inlets are formed from such a void. Although the void above the retort can also be used for forming boreholes into the formation for blasting operations and/or can be used for control of the retort during retorting, it is expensive and time-consuming to excavate such a void.
Consequently, there is the need to develop techniques for avoiding bypassing of oil shale in a retort having an outlet that communicates with the fragmented mass through a side boundary near its bottom, while also avoiding thermal sloughing. Additionally, it is preferable that the number of voids excavated for forming such an in situ oil shale retort be minimized so as to enhance the economics of the overall retorting operation.